Fixes #56 (and by extension astropy/astropy#2356).  It makes more sense, if a file-like object does not have an obvious 'closed' attribute, to just assume it is 'open'

git-svn-id: http://svn6.assembla.com/svn/pyfits/branches/3.2-stable@2683 ed100bfc-0583-0410-97f2-c26b58777a21